Control system for alleviating shock in automatic transmission

ABSTRACT

An accumulator designed to alleviate a so-called N-D select shock is connected in a hydraulic control system for an automatic transmission such that it can also alleviate a 3-4 shift shock. The accumulator includes a piston and a spring biasing the piston. The spring force of the spring is appropriately set such that the piston starts moving against the spring in response to small pressure build-up of a drive range select pressure.

COPENDING RELATED APPLICATIONS

Reference should be made to the following copending U.S. applicationswhich have been assigned to the assignee of the present application.

U.S. application Ser. No. 885,136, filed July 14, 1986 claiming priorityon Japanese Patent Application No. 60-171866 filed on Aug. 6, 1985;

U.S. application Ser. No. 885,135, filed July 14, 1986 claiming priorityon Japanese Patent Application No. 60-154244 filed on July 15, 1985;

U.S. application Ser. No. 890,371, filed July 29, 1986 claiming priorityon Japanese Patent Application No. 60-166646 filed on July 30, 1985;

U.S. application Ser. No. 890,370, filed July 29, 1986 claiming priorityon Japanese Patent Application No. 60-166647 filed on July 30, 1985,

U.S. application Ser. No. 905,078, filed Sept. 9, 1986 claiming priorityon Japanese Patent Applications No. 60-199318 filed on Sept. 11, 1985,and No. 60-199319 filed on Sept. 11, 1985.

U.S. application Ser. No. 893,243, filed Aug. 25, 1986 claiming priorityon Japanese Patent Applications No. 60-171154 filed on Aug. 5, 1985, No.60-171865 filed on Aug. 6, 1985, No. 60-171869 filed on Aug. 6, 1985,and No. 60-197078 filed on Sept. 6, 1985.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for alleviating shocksoccurring within an automatic transmission.

In an automatic transmission, when a driver places a manual valve from aneutral (N) range to a forward drive (D) range, a hydraulic pressure issupplied to a forward drive friction element, thus permittingestablishment of a torque delivery path for forward running of a vehiclewith automatic shift, while when he/she places the manual valve from theN range to a reverse drive (R) range, the hydraulic fluid pressure issupplied to reverse drive friction elements, thus permittingestablishment of a torque delivery path for reverse running with asingle gear ratio. If, upon shifting the manual valve from the N rangeto the D range or to the R range, the supply of the hydraulic pressureto the corresponding friction element or elements is too quick, thetransmission torque of the friction element or elements rises toorapidly to generate substantially great shock. This shock is hereinaftercalled as a N-D select shock if it occurs upon placing the manual valvefrom the N range to the D range and as a N-R select shock if it occursupon placing the manual valve from the N range to the R range. In thecase where a friction element is to be engaged with the above mentionedforward drive friction element kept engaged in order to effect a shiftdepending on running condition of the vehicle, if the supply ofhydraulic fluid pressure to the friction element to be engaged is rapid,a great shift shock occurs due mainly to rotational inertia of theengine.

In order to deal with this problem, it has been proposed in U.S. Pat.No. 4,274,308 to use a common accumulator to suppress the abovementioned two kinds of select shock as well as shift shock. This knownaccumulator is explained along with FIG. 5. It has a stepped piston abiased by a spring b upwards as viewed in FIG. 5. The stepped piston adefines between two different diameter piston ends an intermediatechamber c for receiving a back-up pressure related to the engine load(or engine torque or throttle opening degree). In this known example, aforward drive select pressure, i.e., a line pressure variable inproportion to the engine load and kept supplied to a forward drivefriction element during forward drive. It defines a first end chamber dwhich the smaller diameter piston end is exposed to and a second endchamber e which the larger diameter piston end is exposed to. The firstend chamber d receives a reverse drive select pressure, while the secondend chamber e receives a second speed ratio pressure.

The operation of the accumulator is hereinafter explained.

When the manual valve is placed at the N range, the stepped piston aassumes the illustrated postion under the bias of the spring b sincenone of the three chambers c, d and e are pressurized. If the manualvalve is shited to the R range, the reverse drive select pressureappears and is supplied to the chamber d in such a manner as to push thepiston a against the action of the spring b so that the pressure buildsup gradually in accordance with characteritic determined by the springforce of the spring b. This contributes to alleviation of the N-R selectshock. If, on the other hand, the manual valve is shifted to the Drange, the forward drive pressure appears and is supplied to the chamberc pushing the piston a against the spring b so as to cause a gradualpressure build-up in accordance with the characteristic determined bythe spring force of the spring b, thus contributing to alleviation ofthe N-D select shock. Lastly, if, with the manual valve kept at the Drange, the second pressure appears to effect an upshift to the secondspeed ratio, this pressure is supplied to the chamber e in such a manneras to push back the piston c upwards assisting the action of the springb, thus alleviating the shift shock occurring during 1-2 upshift.

In order to alleviate the shift shock to a satisfactorily low level withthis known accumulator, the setting of the spring b must be such thatthe piston c begins to move upwards against the forward drive selectpressure applied to the chamber c immediately after the supply of secondspeed ratio pressure to the chamber e has begun. This movement of thepiston a must begin even during operating condition with low throttleopening degree setting as will be readily understood from FIG. 6. InFIG. 6, a fully drawn curve α (alpha) shows the variation in enginespecific torque [which is expressed by an equation: (engine outputtorque)/(the maximum engine output torque)] against the variation inthrottle opening degree at 1-2 upshift, while a broken curve β (betha)shows the engine specific torque vs. throttle opening degree at 3-4upshift.

Besides, the upward movement of the piston a must be carried out againstthe forward drive select pressure variable in proportion to the throttleopening degree. In order to assure this upward movement of the piston a,the spring b which is arranged to assist this movement must have arelatively large spring force.

Since, for the preceding rason, the spring force is large, the N-Rselect shock or N-D select shock cannot be alleviated until the pressurebuilds up to a level high enough to push the piston a downwardsovercoming the spring force of the spring b. This shick alleviatingcharacteristic is not satisfactory because the select shock is notsuppressed to sufficiently low level.

An object of the present invention is to provide a control systemwherein an accumulator is operatively disposed therein in order toalleviate not only shift shock, but also select shock to satisfactorilylow levels, respectively.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a control systemfor an automatic transmission which is shiftable to a plurality of speedratios including a predetermined speed ratio other than a first speedratio and a second speed ratio. The control system comprises anaccumulator including a piston defining three chambers which areselectively expandable in volume in response to movement of the piston,and resilient means acting on said piston in assisting movement of saidpiston in one of two directions and in yieldably resisting movement ofsaid piston in the other direction which is opposite to said onedirection. The control system also comprises means for keeping a firsthydraulic fluid pressure applied to first one of said three chambers toact on said piston to urge the same against said resilient means in sucha manner as to decrease the volume of second one of said three chamberswhile a predetermined drive range is selected in the automatictransmission; a friction element to be engaged when the transmissionuphifts to the predetermined speed ratio while said predetermined driverange is selected, said friction element communicating with said secondchamber; means for applying a second hydraulic pressure to said secondchamber to assist the action of said resilient means to cause saidpiston to move in said one direction so as to expand the volume of saidsecond chamber, allowing gradual build-up in pressure applied to saidfriction element; a second friction element communicating with third oneof said three chamber to be enagaged by pressure build-up therein; andmeans for applying a third hydraulic fluid pressure to said third one ofsaid three chambers with the other first and second chambers keptdepressurized, when a second predetermined drive range is selected inthe automatic transmission, to act on said piston to move said pistonagainst the action of said resilient means in said second direction soas to expand the volume of said third chamber, allowing gradual build-uppressure applied to said second friction element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C, when combined side by side, illustrate a controlsystem for an automatic transmission according to the present invention;

FIG. 2 is a schematic view showing the power train of the automatictransmission with an engine and a road wheel of the automotive vehicle;

FIG. 3 is a schematic sectional view of a band brake;

FIG. 4 is a fragmentary sectional view showing second embodimentaccording to the present invention;

FIG. 5 is a sectional view of a known accumulator discussed before; and

FIG. 6 shows specific torque versus throttle opening degreecharacteristic curves of an engine operatively coupled with theautomatic transmission shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1A, 1B, 1C, 2, and 3, there is described a preferredembodiment according to the present which incoporates various featureswhich will be described later in connection with FIGS. 4 to 9.

Referring to FIG. 2, the power train is shown as being connected to anengine with a throttle valve which opens in degrees and road wheelsalthough only one being diagrammatically shown. The power train isdriven by the engine output shaft 1. To transmit engine torque to aninput or turbine shaft 2, a torque converter 3 has a pump impellerconnected to engine output shaft 1, and a turbine runner 3T connected toinput shaft 2. The torque converter 3 also includes a stator 3S. Pumpimepeller 3P is connected to an oil pump O/P to drive the same. Thepower transmission further comprises a first planetary gear set 4, asecond planetary gear set 5, and output shaft 6, and a various kinds offrictional elements which will be described later.

Torque converter 3 is of a so-called lock-up torque converter includinga lock-up clutch 3L which when engaged, mechanically connects the pumpimpeller 3P with the turbine runner 3T, thus eliminating slip takingplace between them. Lock-up clutch 3L assumes a disengaged position whenworking fluid is supplied to a release chamber 3R and then dischargedfrom an apply chamber 3A. Lock-up clutch 3L is disengaged when workingfluid is supplied to the apply chamber 3A and then discharged from therelease chamber 3R.

First planetary gear set 4 is of a simple planetary gear set whichcomprises a sun gear 4S, a ring gear 4R, pinions 4P meshing with them,and a carrier 4P rotatably carrying the pinions 4P. Second planetarygear set 5 is of a simple planetary gear set which comprises a sun gear5S, a ring gear 5R, pinions 5P meshing with them, and a carrier 5Crotatably carrying the pinions 5P.

Carrier 4C is connectable with input shaft 2 via a high clutch H/C, andsun gear 4S is connectable with input shaft 2 via a reverse clutch R/C.Sun gear 4S is adpated to be anchored by a band brake B/B. Carrier 4C isadapted to be anchored by a low & reverse brake LR/B and its reverserotation is prevented by a low one-way clutch LO/C. Ring gear 4R isconnected integrally with carrier 5C which is drivingly connected tooutput shaft 6. Sun gear 5S is connected with input shaft 2. Ring gear5R is connectable with carrier 4C via an overrun clutch OR/C. Toestablish a predetermined drive relation, a forward one-way clutch FO/Cand a forward clutch F/C are arranged between the carrier 4C and thering gear 5R. Engagement of the forward clutch F/C causes the forwardone-way clutch FO/C to connect the ring gear 5R with the carrier 4C inthe reverse rotational direction.

High clutch H/C. reverse clutch R/C, low and reverse brake LR/B, overrunclutch OR/C, and forward clutch F/C are activated, i.e., engaged orapplied, when supplied with hydraulic pressure. Band brake B/B isconstructed as shown in FIG. 3. A housing 10 is formed with a steppedbore including three different diameter bore sections which areconnected one after another as shown in FIG. 3. A stepped piston 8 isreceived in different diameter bore sections having the largest diameterand a diameter smaller than the largest diameter but still larger thanthe smallest. The stepped piston 8 defines on one side of its largerdiameter piston section a second speed servo apply chamber 2S/A and onthe opposite side thereof a third speed servo release chamber 3S/R.Received in the bore section having the smallest diameter is a piston 9.The piston 9 defines on one side thereof a fourth speed servo applychamber 4S/A and on the opposite side thereof where it faces the reduceddiameter piston section of the stepped piston 8 a drain chamber. Thepiston 9 has a plunger slidably extending through the stepped piston 8,the plunger of the piston 9 serving as means for defining a passageadapted to deliver fourth speed pressure P4 to fourth speed servo applychamber 4S/A. The plunger of piston 9 is enlarged in diameter at theremote end portion from the piston to form a shoulder abutting with thatside of the stepped piston 8 exposed to the three speed servo releasechamber 3S/R. The plunger is anchored to a brake band 13. Providedwithin the drain chamber is a spring 11 and provided within the boresection having the largest diameter is a spring 12. Spring 11 has oneend bearing against the stepped piston 8 and opposite end bearingagainst the piston 9 to separate them to have them assume theillustrated position where the shoulder of the plunger is urged to abutwith stepped piston 8. The spring 12 bears against the stepped piston 8to urge the same in the release portion as illustrated in FIG. 3.

With the above construction, when second speed pressure P2 is suppliedto the second servo apply chamber 2S/A, the stepped piston 8 togetherwith the piston 9 are urged to move to the right as viewed in FIG. 3 andtightens the brake band 13 to apply the band brake B/B. In this state,supplying third speed pressure to the third speed servo release chamber3S/R causes the stepped piston 8 to move to the right as viewed in FIG.3 due to a difference in pressure acting area on the opposite sides ofthe stepped piston 8, thus releasing brake band 13 to release theapplication of band brake B/B. In this state, when fourth speed pressureP4 is supplied to the fourth speed servo apply chamber 4S/R, the piston9 is urged to move to the left independent of the stepped piston 8,tightening the brake band 13 to apply the band brake B/B.

The power train shown in FIG. 2 is shiftable into one of first, second,third, and fourth speed ratio during forward travel or into reverse whenselected one or ones of friction elements B/B, H/C, F/C, OR/C, LR/B, andR/C are activated in accordance with a pattern as shown in TABLE 1 incombination with activation of friction elements FO/C and LO/C. In TABLE1, a selected one or ones of servo chambers 2S/A, 3S/R, and 4S/Aassigned with the reference character "o" are supplied with hydraulicpressure, a selected one or ones of friction elements H/C, R/C, LR/B,and RC assigned with the reference character "o" are supplied withhydraulic pressure and activated. The reference character "x" denotesone or ones of friction elements OR/C and LR/B which are to be activatedto effect engine brake running in a particular one of the forward speedratios. When the overrun clutch OR/C is activated, forward one-wayclutch FO/C arranged in parallel with the same is not in operation,while when the low and reverse brake LR/B is activated, low one-wayclutch LO/C arranged in parallel with the same is not in operation.

                                      TABLE 1                                     __________________________________________________________________________    B/B                                                                           2S/A    3S/R                                                                             4S/A                                                                              H/C                                                                              F/C                                                                              FO/C                                                                              OR/C                                                                              LO/C                                                                              LR/B                                                                              R/C                                      __________________________________________________________________________    REV.                             o   o                                        FORW                                                                          1st.              o  o   x   o   x                                            2nd.                                                                              o             o  o   x                                                    3rd.                                                                              o   o      o  o  o   x                                                    4th.                                                                              o   o  o   o  o                                                           __________________________________________________________________________

Referring to FIGS. 1A, 1B and 1C, a pressure regulator valve 20 shown inFIG. 1A comprises a spool 20a biased by a spring 20a to a spring setposition illustrated by the left half thereof as viewed in FIG. 1A, anda plug 20c. Pressure regulator valve 20 regulates hydraulic fluiddischarged to a circuit 71 by an oil pump O/P to generate a hydraulicfluid pressure that is determined by the spring force of spring 20a andthe force biased by plug 20c to spool 20b. This hydraulic pressureresulting from the pressure regulation is used as a servo actuatingfluid pressure and called as "line pressure." The hydraulic fluidpressure building up in circuit 71 acts via a damping orifice 72 on apressure acting area 20d of spool 20b to urge the same to move downwardsas viewed in FIG. 1A against spring 20a. Pressure regulator valve 20 isformed with four ports 20e, 20f, 20g, and 20h which are arranged suchthat they are opened or closed in response to which position spool 20bassumes during its stroke. Port 20e is connected to circuit 71 and itstarts to communicate with ports 20h and 20f and increases itscommunication with them as spool 20b moves downwards from the spring setposition as illustrated by the left half thereof in FIG. 1A. Port 20fdecreases its communication with port 20g serving as a drain port asspool 20b moves downwards from the illustrated spring set position, andit starts to communicate with port 20e when it has interrupted itscommunication with drain port 20g. Port 20f is connected to a volumecontrol actuator 75 for oil pump O/P via a circuit 74 provided with ableeder orifice 73. Oil pump O/P is of a variable volume vane pumpdriven by the engine and its volume decreases to become small when thepressure supplied to volume control actuator 75 exceeds a predeterminedvalue. Plug 20c of pressure regulator valve 20 has a bottom end servingas a pressure acting area which receives a modifier pressure from acircuit 76, and has a pressure acting area 20i a reverse select pressurefrom a circuit 77. Thus, plug 22c applies to spool 20b an upward forceresulting from forces created by reception of the pressure modifierpressure or the reverse select pressure.

Initially, the pressure regulator valve 20 assumes the state asillustrated by the left half thereof as viewed in FIG. 1A. When the oilpump O/P starts discharging oil, the oil discharged flows into thecircuit 71. When the spool 20b stays in the positon as illustrated bythe left half thereof, the pressure rises within the circuit 71 becauseno oil is drained therefrom. This pressure is supplied via an orifice 72to a pressure acting area 20d to act thereon, urging the spool 20bdownwards against the spring 20a until the port 20e communicates withthe port 20h. As a result, the above mentioned pressure is drained atthe port 20h and thus drops, allowing the spool 20b for being pushedback by the spring 20a. Repeating these operations one after anothercauses the pressure regulator valve 20 to adjust the pressure within thecircuit 71 (i.e., hereinafter called as a line pressure) to basically avalue corresponding to the spring force of the spring 20a. The plug 20c,on the other hand, is subject to a modifier pressure acting upon thebottom end thereof and urges the spool 20h upwards after coming intoabutting contact with the spool 20b as illustrated by the right halfthereof as viewed in FIG. 1A in such a manner as to assist the action ofthe spring 20a. As will be later described, the modifier pressureappears when each of the drive ranges except the reverse drive range isselected and rises in proportion to the engine load (which correspondsto the engine output torque). Thus, the above mentioned line pressurevaries such that it rises with an increase in the engine load.

When the reverse drive range is selected, a reverse select pressure ashigh as the line pressure is supplied through a circuit 77 to thepressure regulator valve 20 to act on the plug 20c in a direction tourge the same upwards, urging the spool 20b in the direction assistingthe spring force of the spring 20a after the plug 20c has been broughtinto abutting contact with the spool 20b. This causes the line pressureto be boosted up to a relatively high predetermined value desired forreverse drive.

As the engine speed increases, the amount of oil discharged by the oilpump O/P increases. After the engine speed has exceeded a certain level,an overabundance amount of oil is discharged by the oil pump O/P,causing the pressure within the circuit 71 to rise above the valueadjusted. This pressure rise causes the spool 20b to move downwardsbeyond the equilibrium position as illustrated by the right half thereofas viewed in FIG. 1A where the communication of the port 20f with thedrain port 20g prevously established is blocked and the communication ofthe port 20f with the port 20h previously blocked is established,allowing the discharge of oil from the port 20e via the port 20f at ableed 73 in a flow restricted manner, leaving a feedback pressurecreated within the circuit 74. This feedback pressure rises as therevolution speed of the oil pump O/P increases, causing the actuator 75to bring down the eccentric amount (i.e., the pump capacity) of the oilpump O/P. As a result, the discharge amount of the oil pump O/P is keptconstant while the revolution speed of the oil pump O/P is higher thanthe certain level, causing no power loss of the engine which wouldresult if the oil pump were activated to discharge the overabundanceamount of oil.

The line pressure created within the circuit 71 in the above describedmanner is distributed through a line pressure circuit 78 to a pilotvalve 26, a manual selector valve 36, an accumulator control valve 70and a servo release pressure accumulator 66.

The pilot valve 26 comprises a spool 26b biased by a spring 26a to aposition as illustrated by the upper half thereof as viewed in FIG. 1A.The spool 26b has its remote end from the spring 26a exposed to achamber 26c. The pilot valve 26 also includes a drain port 26d and isconnected to a pilot pressure circuit 79 having therein a strainer S/T.The spool 26b is formed with a connecting passage 26e which allowing thetransmission of the pressure from the pilot pressurre circuit 79 to thechamber 26c. As the pressure within the chamber 26c rises, the spool 26bis urged for rightward movement as viewed in FIG. 1A. This rightwardmovement of the spool 26b allows the pilot pressure circuit 79 to switchits connection from the line pressure circuit 78 to the drain port 26d.

With the pilot valve 26b held in the position as illustrated by theupper half thereof as viewed in FIG. 1A, supplying the line pressurefrom the circuit 78 to the pilot valve 26 causes a rise in pressurewithin the circuit 79. This rise in pressure is supplied via theconnection passage 26e to the chamber 26c, causing the rightwardmovement of the spool 26b as viewed in FIG. 1A. This rightward movementof the spool 26b beyond the equilibrium state position as illustrated bythe lower half thereof as viewed in FIG. 1A causes the circuit 79 toblock its communication with the circuit 78 and at the same time openits communication with the drain port 26d. This results in a drop inpressure within the circuit 79, allowing the spool 26b to be pushed backby the spring 26a, causing the pressure within the circuit 79 to riseagain. Thus, the pilot valve 26 reduces the line pressure from thecircuit 78 down to a constant value that is determined by the springforce of the spring 26a and outputs the results to the circuit 79 as thepilot pressure.

This pilot pressure is supplied through the circuit 79 to the pressuremodifier valve 22, duty solenoids 24, 34, a lock-up control valve 30, aforward clutch control valve 30, a forward clutch control valve 46, ashuttle valve 32, first, second and third shift solenoids 42, 44 and 60,and a shuttle valve 56.

The duty solenoid 24 comprises a coil 24a, a spring 24d and a plunger24b. When the coil 24a is turned ON (i.e., when electric current passesthrough the coil 24a), the plunger 24b is electromagnetically drawnagainst the spring 24d to an open position where a circuit 81 that isconnected to the circuit 79 via an orifice 80 is allowed to communicatewith a drain port 24c. Under the control of a computer, not illustrated,the coil 24a of the duty solenoid 24 is turned ON intermittently. Theratio of ON time to the period which is constant (i.e., duty cycle) iscontrolled, causing pressure within the circuit 81 to vary in dependenceon the duty cycle. The duty cycle ranges from 0% to 100% such thatduring operation with each of the drive ranges except that reverse driverange being selected, the duty cycle is decreased as the engine load(for example, the engine throttle opening degree) increases, and theduty cycle is set as 100% when the reverse drive range is selected. Whenthe duty cycle is set as 100%, the control pressure within the circuit81 is zero, whereas when it is set as 0%, the control pressure takes themaximum value.

The pressure modifier valve 22 comprises a spring 22a and a spool 22bwhich is biased downwards as viewed in FIG. 1A by means of the springforce of the spring 22a and a control pressure from the circuit 81. Thepressure modifier valve 22 is provided with an outlet port 22c connectedto the circuit 76, an inlet port 22d connected to the pilot pressureport 79, and a drain port 22e. The spool 22b has its remote end from thespring 22a exposed to a chamber 22f which the circuit 76 is connectedto. The arrangement is such that upon the spool assuming a position asillustrated by the left half thereof as viewed in FIG. 1A, the port 22cbecomes out of communication with the ports 22d and 22e.

In the pressure modifier valve 22, the spool 22b is urged downwards, asviewed in FIG. 1A, upon being subject to the spring force by the spring22a and the control pressure supplied thereto from the circuit 81,whereas it is urged upwards upon being subject to the output pressurefrom the outlet port 22c. The spool 22b assumes the eqilibrium positionwhere the forces acting on the spool 22b balance. If the upward forcedue to the output pressure acting on the spool 22b is insufficient foropposing to the downward forces due to the spring force and the controlpressure acting on the spool 22b, the spool 22b tends to move downwardsbeyond the equilibrium position as illustrated by the left half thereofas viewed in FIG. 1A. This downward movement of the spool 22b uncoversthe port 22d to allow it to communicate with the port 22c, allowing thesupply of pilot pressure from the circuit 79 to the port 22c, thuscausing a rise in the output pressure. If the upward force due to theoutput pressure acting on the spool 22b is excessively large foropposing to the downward forces, the spool 22b moves upwards toward aposition as illustrated by the right half thereof as viewed in FIG. 1A.In this position of the spool 22b, the port 22c is allowed tocommunicate with the drain port 22e, causing a drop in the outputpressure. After repeating these operations, the pressure modifier valve22 adjusts the output pressure from the port 22c to a valuecorresponding to the sum of the spring force of the spring 22a and theforce due to the control pressure from the circuit 81, and supplies theoutput pressure, as a modifier pressure, through the circuit 76 to theplug 20c of the pressure regulator valve 20. With this modifier pressureresulting from amplification of the control pressure by the spring forceof the spring 22a, the line pressure is controlled such that it rises asthe engine load increases during operation with each of the drive rangesexcept the reverse drive range being selected because the controlpressure rises as the engine load increases although it is zero duringoperation with the reverse drive selected.

The torque converter regulator valve 28 comprises a spring 28a, and aspool 28b biased by the spring to a position as illustrated by the righthalf thereof as viewed in FIG. 1A. It is formed with ports 28c and 28dwhich are kept communicating with each other during the stroke of thespool 28b between the position as illustrated by the left half thereofand the position as illustrated by the right half thereof as viewed inFIG. 1A. As it moves upwards from the position as illustrated by theleft half thereof as viewed in FIG. 1A, the spool 28b begins coveringthe port 28d and uncovering the port 28e, reducing the degree of thecommunication of the port 28c with the port 28d and increasing thedegree of communication of the port 28c with the port 28e. In order tocontrol the stroke of the spool, the spool 28b has its remote end fromthe spring 28a exposed to a chamber 28f and it has formed therethrough aconnecting passage with which the chamber 28f communicates always withthe port 28c. The port 28c is connected to predetermined portions to belubricated via a relief valve 82 and also to the lock-up control valve30 via a circuit 83. The port 28d is connected via a circuit 84 to thepressure regulator valve 20 at the port 20h. The port 28e is alsoconnected to the lock-up control valve 30 via a circuit 85. This circuit85 is provided with an orifice 86 and it has a portion thereof betweenthe orifice 86 and the port 28e connected to the circuit 83 via anorifice 87 and also to an oil cooler 89 and parts 90 to be lubricatedvia a circuit 88.

Initially, the torque converter regulator valve 28 assumes the positionas illustrated by the right half thereof as viewed in FIG. 1A. Underthis condition, oil supplied thereto from the port 20h of the pressureregulator valve 20 via the circuit 84 is allowed to pass through thecircuit 83 to the torque converter 3 in the manner described later,causing a torque converter supply pressure to build up at the port 28d.This torque converter supply pressure is supplied to the chamber 28f viathe connection passage 28g, urging the spool 28b upwards, as viewed inFIG. 1A against the spring 28a. When the spool 28b moves upwards beyondthe position as illustrated by the left half thereof as viewed in FIG.1A, it begins to uncover the port 28e, allowing the discharge of oilthrough the port 28e and circuit 88. This causes the torque convertersupply pressure to be adjusted to a value determined by the spring forceof the spring 28a. The oil discharged from the circuit 88 is directedtoward the lubrication parts 90 after being subject to cooling by theoil cooler 89. If the torque converter supply pressure exceeds the abovementioned value as a result of the above mentioned pressure regulation,the relief valve 82 opens to relieve the excessive pressure toward thelubrication parts so as to avoid deformation of the torque converter 3.

The lock-up control valve 30 comprises a spool 30a and a plug 30b whichare arranged along an axis. When the spool 30a assumes a position asillustrated by the right half thereof as viewed in FIG. 1A, the circuit83 is allowed to communicate with a circuit 91 leading to a torqueconverter release chamber 3R, while when it moves downward to a positionas illustrated by the left half thereof as viewed in FIG. 1A, thecircuit 83 is now allowed to communicate with the circuit 85. When thespool 30a moves downwards beyond the position as illustrated by the lefthalf thereof as viewed in FIG. 1A, the circuit 91 is allowed tocommunicate with a drain port 30c. In order to control the stroke of thespool 30a, the spool 30a has its remote end from the plug 30b exposed toa chamber 30d, and the plug 30b has its remote end from the spool 30aexposed to a chamber 30e which the pressure within the circuit 91 issupplied thereto via an orifice 92. There is provided a circuit 91extending from a torque converter apply chamber 3A which is connected tothe circuit 85 at a portion between the orifice 86 and the lock-upcontrol valve 30. The plug 30b is subject to a downward force resultingfrom the pilot pressure acting thereon from the circuit 79 via anorifice 94, preventing pulsation of the spool 30a.

In the lock-up control valve 30, the stroke of the spool 30a iscontrolled by pressure supplied to a chamber 30d. When this pressure issufficiently high, the spool 30a assumes a position as illustrated bythe right half thereof as viewed in FIG. 1A. In the position of thespool 30a, the oil from the circuit 83 which is under pressureregulation by the torque converter regulator valve 28 passes through thecircuit 91, release chamber 3R, apply chamber 3A, circuit 93 and circuit85 to the circuit 88 where it is discharged. Under this condition, thetorque converter 3 performs power transmission in its converter state.As the pressure within the chamber 30d drops, the spool 30a is urged fordownward movement, as viewed in FIG. 1A, by means of the plug 30b due topressure acting thereon via the orifices 92 and 94. When, during thisdownward movement, the spool 30a moves downward beyond the position asillustrated by the left half thereof as viewed in FIG. 1A, the oil underregulation from the circuit 83 passes through the circuits 85, 93, applychamber 3A, release chamber 3R and circuit 91 to the drain port 30cThus, the torque converter performs the power transmission in a slipstate with a rate controlled in proportion to a drop in the pressurewithin the chamber 30d of the torque converter 3. Further drop in thepressure within the chamber 30d causes further downward movement of thespool 30a beyond this state. This causes the circuit 91 to fullycommunicate with the drain port 30c, bringing down the pressure withinthe release chamber 3R to zero, allowing the torque converter 3 toperform the power transmission in a lock-up state.

The shuttle valve 32 is designed to effect stroke control of the lock-upcontrol valve 30 as well as that of the forward clutch control valve 46later described. It includes a spool 32b biased by a spring 32a to aposition as illustrated by the lower half thereof as viewed in FIG. 1A.This spool 32b is movable to a position as illustrated by the upper halfthereof as viewed in FIG. 1A in response to pressure within a chamber32c. When the spool 32b assumes the position as illustrated by the lowerhalf thereof as viewed in FIG. 1A, the shuttle valve 32 allows thecircuit 95 from the chamber 30d to communicate with the pilot pressurecircuit 79 and a circuit 96 from a chamber 46a of the forward clutchcontrol valve 46 to communicate with a circuit 97 from a duty solenoid34. When the spool 32b moves to the position as illustrated by the upperhalf thereof, the shuttle valve 32 allows the circuit 95 to communicatewith the circuit 97 and the circuit 96 to communicate with the circuit79.

The duty solenoid 34 comprises a coil 34a, a spring 34d and a plunger34b biased to a close position by the spring 34d. When the coil 34a isturned ON (i.e., when electric current passes through the coil 24a), theplunger 34b is electromagnetically drawn against the spring 34d to anopen position where a circuit 79 that is connected to the circuit 79 viaan orifice 98 is allowed to communicate with a drain port 34c. Under thecontrol of a computer, not illustrated, the coil 34a of the dutysolenoid 34 is turned ON intermittently. The ratio of ON time to theperiod which is constant (i.e., duty cycle) is controlled, causingpressure within the circuit 97 to vary in dependence on the duty cycle.In the case where the shuttle valve 32 assumes the position asillustrated by the upper half thereof and the control pressure withinthe circuit 97 is used to effect the stroke control of the lock-upcontrol valve 30, the duty cycle of the solenoid 34 is determined asfollows. That is, the duty cycle should be 0% to allow the controlpressure within the circuit 97 to increase as high as the pilot pressurewithin the cicuit 79 when the engine operates with heavy load at lowspeeds where the torque multiplying function of the torque converter 3and the torque variation absorbing function thereof are required. Underthis condition, the control pressure supplied to the chamber 30d urgesthe spool 30a to the position as illustrated by the right half thereof,rendering the torque converter 3 to operate in the torque converterstate as desired. As the requirement degree of the above mentioned twofunctions imposed on the torque converter 3 decreases, the duty cycleshould be increased to bring down the control pressure, rendering thetorque converter 3 to operate in the desired slip state. When the engineoperates with light load at high speeds where the above mentionedfunctions of the torque converter are not required, the duty cycleshould be 100% to bring down the control pressure to zero, allowing thetorque converter 3 to operate in the lock-up state.

In the case where the shuttle valve 32 assumes the position asillustrated by the lower half thereof as viewed in FIG. 1A and thecontrol pressure within the circuit 97 is used to effect the strokecontrol of the forward clutch control valve 46, the duty cycle of thesolenoid 34 is determined in such a manner later described that N-Dselect shock is alleviated and creep is prevented.

The manual selector valve 36 comprises a spool 36a which is movabledepending on manual select operation of a driver to a park (P) range, areverse (R) range, a neutral (N) range, a forward automatic drive (D)range, a forward second speed engine brake (II) range, a forward firstspeed ratio engine brake (I) range. Selecting one of the above mentionedranges causes the line pressure line pressure circuit 78 to communicatethe corresponding one of the output ports 36 D, 36 II, 36 I, and 36 R inaccordance with the pattern shown by the following table.

                  TABLE 2                                                         ______________________________________                                               Range                                                                  Port     P     R         N   D       II  I                                    ______________________________________                                        36 R           o                                                              36 D                         o       o   o                                    36 II                                o   o                                    36 I                                     o                                    ______________________________________                                    

In the above TABLE, the reference character "o" denotes the particularport which communicates with the line pressure circuit 78, while theother ports which are not denoted by this reference character aredrained.

The first shift valve 38 comprises a spring 38a and a spool 38b biasedby the spring 38a to a position as illustrated by the left half thereofas viewed in FIG. 1B. This spool 38b assumes a position as illustratedby the right half thereof as viewed in FIG. 1A when a pressure issupplied to a chamber 38c. When the spool 38b assumes the position asillustrated by the left half thereof as viewed in FIG. 1B, the firstshift valve 38 allows a port 38d to communicate with a drain port 38e, aport 38f to communicate with a port 38g, and a port 38h to communicatewith a port 38i. When the spool 38b assumes the position as illustratedby the right half thereof as viewed in FIG. 1B, the first shift valve 38allows the port 38d to communicate with a port 38j, the port 38f tocommunicate with a port 38k, and the port 38h to communicate with a port38l.

The second shift valve 40 comprises a spring 40a and a spool 40b biasedby the spring 40a to a position as illustrated by the left half thereofas viewed in FIG. 1B. This spool 38b assumes a position as illustratedby the right half thereof as viewed in FIG. 1A when a pressure issupplied to a chamber 40c. When the spool 40b assumes the position asillustrated by the left half thereof as viewed in FIG. 1B, the secondshift valve 40 allows a port 40d to communicate with a drain port 40e, aport 40f to communicate with a port 40g, and a port 40h to communicatewith a drain port 40i via an orifice. When the spool 40b assumes theposition as illustrated by the right half thereof as viewed in FIG. 1B,the second shift valve 40 allows the port 40d to communicate with theport 40j, the port 40f to communicate with the port 40e, and the port40h to communicate with a port 40k.

The spool positions of the first and second shift valves 38 and 40 arecontrolled by a first shift solenoid 42 and a second shift solenoid 44,respectively. These shift solenoids comprise coils 42a and 44a,respectively, plungers 42b and 44b, respectively, and springs 42d and44d, respectively. The first shift solenoid 42 is connected to the pilotpressure circuit 79 via an orifice 99, and it blocks communicationbetween a circuit 100 leading to a chamber 38c and a drain port 42c whenthe coil 42a is turned ON (i.e., when the current passes through thecoil 42a), allowing a control pressure within the circuit 100 toincrease as high as the pilot pressure, uring the first shift valve 38to move to the position as illustrated by the right half thereof asviewed in FIG. 1B. The second shift solenoid 44 is connected to thepilot pressure circuit 79 via an orifice 101, and it blockscommunication between a circuit 102 leading to a chamber 40c and a drainport 44c when the coil 44a is turned ON (i.e., when the current passesthrough the coil 44a), allowing a control pressure within the circuit102 to increase as high as the pilot pressure, urging the second shiftvalve 40 to move to the position as illustrated by the right halfthereof as viewed in FIG. 1B.

The first to fourth speed ratios are established depending on variouscombinations of ON and OFF of the shift solenoids, i.e., variouscombinations of upshift and downshift positions of the shift valves 38and 40, in accordance with the pattern shown in the following TABLE 3.

                  TABLE 3                                                         ______________________________________                                                      Speed                                                           Element         1st    2nd       3rd  4th                                     ______________________________________                                        1st Shift Solenoid 42                                                                         ON     OFF       OFF  ON                                      1st Shift Valve 38                                                                            o      x         x    o                                       2nd Shift Solenoid 44                                                                         ON     ON        OFF  OFF                                     2nd Shift Valve 40                                                                            o      o         x    x                                       ______________________________________                                    

In the above TABLE, the reference character "o" represents the statewhere the shift valve is in the position as illustrated by the righthalf thereof as viewed in FIG. 1B, while the reference character "x"represents the state where the shift valve is in the position asillustrated by the left half thereof as viewed in FIG. 1B. The ON andOFF of the shift solenoids 42 and 44 are determined by the computer, notillustrated, versus vehicle speed and engine load in accordance with apredetermined shift pattern so as to establish an appropriate speedratio for the vehicle speed and the engine load.

The forward clutch control valve 46 comprises a spool 46b. Forpreventing the pulsation of the spool 46b, the pilot pressure suppliedto the forward clutch control valve 46 from the circuit 79 via anorifice 103 acts on the spool 46b in a downward direction as viewed inFIG. 1A. The spool 46b ia subject to another downward force, too,created by the actuating pressure for the forward clutch F/C supplied,as a feedback pressure, to the clutch control valve 46 from a circuit105 via an orifice 104 to act on the spool 46b. The spool 46b will moveto a position where the above mentioned downward forces balance with anupward force due to the pressure within the chamber 46a. When the spool46b assumes the position as illustrated by the right half thereof asviewed in FIG. 1A, the circuit 105 is allowed to communicate with adrain port 46c, whereas when it assumes the position as illustrated bythe left half thereof, the circuit 105 is allowed to communicate with acircuit 106. The circuit 105 is provided with a one-way orifice 107which has a throttle effect only upon the hydraulic pressure directedtoward the forward clutch F/C. The circuit 106 is connected to the port36 D of the manual selector valve 36.

The 3-2 timing valve 48 comprises a spring 48a and a spool 48b biasedtoward a position as illustrated by the left half thereof as viewed inFIG. 1C where a port 48c is allowed to communicate with a port 48dhaving an orifice 48f. When a pressure within a chamber 48e is highenough to urge the spool 48b to the position as illustrated by the righthalf thereof as viewed in FIG. 1C, the communication between the ports48c and 48d is blocked.

The 4-2 relay valve 50 comprises a spring 50a and a spool 50b biased bythe spring 50a to a position as illustrated by the left half thereof asviewed in FIG. 1B where a port 50c is allowed to communicate with adrain port 50d having an orifice therein. When a pressure within achamber 50e is high enough to urge the spool 50b to a position asillustrated by the right half thereof as viewed in FIG. 1B, the port 50cis allowed to communicate with a port 50f.

The 4-2 sequence valve 52 comprises a spring 52a and a spool 52b biasedby the spring 52a to a position as illustrated by the right half thereofas viewed in FIG. 1B where a port 52c is allowed to communicate with adrain port 52d having therein an orifice. When a pressure within achamber 52e is high enough to urge the spool 52b to a position asillustrated by the left half thereof as viewed in FIG. 1B, the port 52cis allowed to communicate with a port 5f.

The I range pressure reduction valve 54 comprises a spring 54a and aspool 54b biased by the spring 54a to a position as illustrated by theright half thereof as viewed in FIG. 1C. It is formed with ports 54c and54d which are allowed to communicate with each other when the spool 54bassume the position as illustrated by the right half thereof. It is alsoformed with a drain port 54e which begins to communicate with the port54c when the spool 54b has moved upwards beyond a position asillustrated by the left half thereof as viewed in FIG. 1C where thespool 54d completely covers the port 54d. The spool 54b has its remoteend from the spring 54a exposed to a chamber 54f connected to the port54c via an orifice 108. Initially, the I range pressure reduction valve54 assumes the position as illustrated by the right half thereof asviewed in FIG. 1C where supplying the port 54d with a pressure causesthe pressure to appear, as an output pressure, at the port 54c. Thisoutput pressure acts via the orifice 108 upon the bottom end, as viewedin FIG. 1C of the spool 54b, urging the spool 54b for upward movement asthe output pressure rises. When, during this upward movement, the spool54b moves upwards beyond the position as illustrated by the left halfthereof as viewed in FIG. 1C, the port 54c begins to communicate withthe drain port 54e, causing a drop in the output pressure from the port54c. This pressure drop causes downward movement of the spool 54b. Whenthe spool moves downwards beyond the position as illustrated by the lefthalf thereof as viewed in FIG. 1C, the port 54c begins to communicatewith the port 54d, causing an increase in the outoput pressure from theport 54c. Repeating these operations results in providing the outputpressure from the port 54c having a constant value that is determined bythe spring force of the spring 54a.

The shuttle valve 56 comprises a spring 56a and a spool 56b biased bythe spring 56a to a position as illustrated by the left half thereof asviewed in FIG. 1C. The spool 56b keeps on staying in this position aslong as a chamber 56g is supplied with a pressure. Under a conditionwhere there is no supply of pressure to the chamber 56g, when an upwardforce due to the pressure from the port 56c and applied to the spool 56bis higher than a predetermined value, the spool 56b is urged to moveupwards to a position as illustrated by the right half thereof as viewedin FIG. 1C. In the position as illustrated by the left half thereof asviewed in FIG. 1C, a port 56d is allowed to communicate with a circuit109 from a third shift solenoid 60, and a port 56e is allowed tocommunicate with a drain port 56f. In the position as illustrated by theright half thereof as viewed in FIG. 1C, the port 56d is allowed tocommunicate with the pilot pressure circuit 79 and the port 56e isallowed to communicate with the circuit 109.

The third shaft solenoid 60 comprises a coil 60a, a plunger 60b and aspring 60d. When the coil 60a is turned ON (i.e., when the currentpasses through the coil 60a), the plunger 60b is urged against thespring 60d to assume a position where the circuit 109 connected via anorifice 110 to the pilot pressure circuit 79 is prevented fromcommunicating with a drain port 60c, causing a control pressure withinthe circuit 109 to increase as high as the pilot pressure. ON and OFF ofthe third shift solenoid 60 are determined by the computer, notillustrated, in a manner later described.

The overrun clutch control valve 58 comprises a spring 58a and a spool58b biased by the spring 58a to a position as illustrated by the lefthalf thereof as viewed in FIG. 1C. This spool 58b moves to a position asillustrated by the right half thereof as viewed in FIG. 1C when achamber 58c is supplied with a pressure. In the position as illustratedby the left half thereof as viewed in FIG. 1C, the spool 58b allows aport 58d to communicate with a drain port 58e, and a port 58ftocommunicate with a port 58g. In the position as illustrated by the righthalf thereof as viewed in FIG. 1C, the spool 58ballows the port 58d tocommunicate with a port 58h and the port 58f to communicate with thedrain port 58e.

The overrun clutch pressure reduction valve 62 comprises a spring 62aand a spool 62b biased by the spring 62a to a position as illustrated bythe left half thereof as viewed in FIG. 1C. This spool 62b is held inthis position under a downward force, as viewed in FIG. 1C, treated whena port 62c is supplied with a pressure. Under a condition where there isno pressure inflow from a port 62c, supplying a port 62d with hydraulicpressure causes an increase in output pressure from a port 62e. Thisoutput pressure is fed back to a chamber 62f, urging the spool 62b forupward movement as viewed in FIG. 1C. When the output pressure attains avalue corresponding to the spring force of the spring 62a, the spool 62bassumes a position as illustrated by the right half thereof as viewed inFIG. 1C where the communication between the ports 62d and 62e isblocked. As a result, the overrun clutch reduction valve 62 effectspressure reduction to adjust the output pressure from the port 62e at aconstant value that is determined by the spring force of the spring 62a.

The second speed ratio servo apply pressure accumulator 64 comprises astepped piston 64a biased by a spring 64b to a position as illustratedby the left half thereof as viewed in FIG. 1B. The stepped piston 64ahas a shoulder thereof exposed to a chamber 64c open to the ambientatmosphere. It has a small diameter piston end and a large diameterpiston end exposed to sealed chambers 64d and 64e, respectively.

The third speed ratio servo release pressure accumulator 66 comprises astepped piston 66a biased by a spring 66b to a position as illustratedby the left half thereof as viewed in FIG. 1C. The stepped piston 66ahas a shoulder exposed to a chamber 66c which the line pressure circuit78 is connected to. It has a small diameter piston end a large diameterpiston end exposed to sealed chambers 66d and 66e, respectively.

The fourth speed ratio servo apply pressure accumulator 68 comprises astepped piston 68a biased by a spring 68b to a position as illustratedby the left half thereof as viewed in FIG. 1C. The stepped piston 68ahas a shoulder exposed to a sealed chamber 68c. It has a small diameterpiston end and a large diameter piston end exposed to sealed chambers68d and 68e, respectively.

The accumulator control valve 70 comprises a spring 70a and a spool 70bbiased by the spring 70a to a position as illustrated by the left halfthereof as viewed in FIG. 1A. The spool 70b has its remote end from thespring 70a exposed to a chamber 70c which the control pressure issupplied to from the circuit 81. In its position as illustrated by theleft half thereof as viewed in FIG. 1A, the spool 70b allows an outletport 70d to communicate with a drain port 70e. When the spool 70b movesupwards to a position as illustrated by the right half thereof as viewedin FIG. 1A in response to a rise in the control pressure supplied to thechamber 70c, the spool 70b allows the outlet port 70d to communicatewith the line pressure circuit 78. The outlet port 70d is connected tothe chambers 64d and 68c of the accumulators 64 and 68, respectively,via a circuit 111, and it is also connected to a chamber 70f receivingtherein the spring 70a.

The spool 70b of the accumulator control valve 70 is urged to moveupwards, as viewed in FIG. 1A, further beyond a position as illustratedby the right half thereof as viewed in FIG. 1B in response to thecontrol pressure supplied to the chamber 70c upon selecting one of thedrive ranges excluding the reverse drive. In this position, the linepressure from the circuit 78 is allowed to output to the circuit 111.When the pressure within the circuit 111 increases a value correspondingto the above mentioned control pressure, the spool 70b assumes theposition as illustrated by the right half thereof as viewed in FIG. 1A.In this manner, the hydraulic pressure within the circuit 111 isadjusted to the value corresponding to the control pressure. Since, aspreviously described, the control pressure increases in proportion toengine load during operation with one of drive ranges excluding thereverse drive range, the hydraulic pressure within the chambers 64d and68c of the accumulators 64 and 68 supplied thereto from the circuit 111will increase in proportion to engine output torque. Upon selecting thereverse drive range, the control pressure is zero so that no hydraulicpressure is output to the circuit 111.

Hereinafter, the hydraulic fluid pressure network is further described.A circuit 106 extending from the port 36 D of the manual selector valve36 has middle portions connected to the port 38g of the first shiftvalve 38 and the port 40g of the second shift valve 40. It has a branchcircuit 112 connected to the port 56c of the shuttle valve 56 and alsoto the port 58g of the overrun clutch control valve 58. The port 38f ofthe first shift valve 38 is connected via a circuit 113 to the port 50fof the 4-2 relay valve 50 and it is also connected via a one-way orifice114 to the accumulator's chamber 64e and the second speed ratio servoapply chamber 2S/A, and the port 50f is connected via the circuit 115 tothe chamber 32c of the shutter valve 32, too. Furthermore, the port 38hof the first shift valve 38 is connected via a circuit 116 to thechamber 50e of the 4-2 relay valve 50 and the port 58h of the overrunclutch control valve 58, while the port 50c of the 4-2 relay valve 50 isconnected via a circuit 117 to the port 40k of the second shift valve40. The ports 38k and 38l of the first shift valve 38 and the port 40fof the second shift valve 40 are connected via a circuit 118 to the highclutch H/C. The circuit 118 is provided with one-way orifices 119 and120 arranged as directed to the opposite directions. Separated from thecircuit 118 at a portion between these orifices 119 and 120 and the highclutch H/C is a branch circuit 121 which is connected via a one-wayorifice 122 to the third speed ratio servo release chamber 3S/R and theaccumulator chamber 66e. There is a circuit 123 connected to the circuit121 bypassing the orifice 122, and the 3-2 timing valve 48 forms part ofthis circuit 123 with their ports 48c and 48d connected within thecircuit 123. Separated from the circuit 121 at a portion between theone-way orifice 122 and the third speed ratio servo release chamber 3S/Ris a circuit 124 which is connected to the chamber 52e of the 4-2sequence valve 52. The ports 52c and 52f of the 4-2 sequence valve 52are connected to the port 38i of the first shift valve 38 and the port40h of the second shift valve 40, respectively.

The first shift valve 38 has its port 38j connected via a circuit 125 tothe port 40d of the second shift valve 40, and its port 38d connectedvia a circuit 126 to one of two inlet ports of a shuttle ball 127. Theother inlet port of the shuttle ball 127 is connected to a circuit 128which is connected at one end to the port 36 R of the manual selectorvalve 36 where the bore mentioned circuit 77 is connected and which isconnected on the other end to the reverse clutch R/C and the accumulatorchamber 68d via a one-way orifice 129. The shuttle ball 127 has itsoutlet port connected via a circuit 130 to the low and reverse brakeLR/B. The port 40j of the second shift valve 40 is connected via acircuit 131 to the port 54c and the chamber 54f of the I range pressurereduction valve 54. The port 54d of the I range pressure reduction valve54 is connected via a circuit 132 to the port 36 I of the manualselector valve 36.

The shuttle valve 56 has its port 56e connected via a circuit 133 to thechamber 48e of the 3-2 timing valve 48, and its port 56d connected via acircuit 134 to the chamber 58c of the overrun clutch control valve 58.The port 58d of the overrun clutch control valve 58 is connected via acircuit 135 to the accumulator chamber 66d, and also to the accumulatorchamber 68e and the fourth speed ratio servo apply chamber 4S/A via aone-way orifice 136. The port 58f of the overrun clutch valve 58 isconnected via a circuit 137 to the port 62f of the overrun clutchreduction valve 62. The overrun clutch pressure reduction valve 62 hasits port 62e connected via a circuit 138 to the overrun clutch OR/C. Acheck valve 139 is connected between the circuits 137 and 138. The port62c of the overrun clutch pressure reduction valve 62 is connected via acircuit 140 to the port 36 II of the manual selector valve 36 and thechamber 56g of the shuttle valve 56.

The operation of the control system is hereinafter described.

The pressure regulator valve 20, pressure modifier valve 22, and dutysolenoid 24 operate in the previously described manner so that the oildischarged by the oil pump O/P is regulated to provide the line pressurethat rises in proportion to the engine output torque during the driveranges excluding the reverse drive range and that is kept constantduring the reverse drive range, and this line pressure is supplied tothe circuit 78. This line pressure reaches the pilot valve 26, manualselector valve 36, accumulator control valve 70, and accumulator 66,keeping the accumulator 66 at the position as illustrated by the righthalf thereof as viewed in FIG. 1B. During the drive ranges excluding thereverse drive range, the accumulator control valve 70 supplies, via thecircuit 111, the chambers 64d and 68c of the accumulators 64 and 68 withan accumulator backup pressure (i.e., the line pressure) that isvariable in proportion to the engine output torque, urging theseaccumulators to assume the positions as illustrated by the right halvesthereof as viewed in FIGS. 1B and 1C, respectively. During the reversedrive, the accumulator control valve 70 brings down the accumulatorbackup pressure to zero, allowing the accumulators 64 and 68 to assumethe positions as illustrated by the left halves thereof as viewed inFIGS. 1B and 1C, respectively. The pilot valve 26 always outputs theconstant pilot pressure to the circuit 79.

P, N RANGE

When the driver places the manual selector valve 36 at the P range or Nrange, all of the ports 36 D, 36 II, 36 I and 36 R of the manualselector valve 36 serve as drain ports. Since the line pressure does notcome out of these ports, the forward clutch F/C, high clutch H/C, bandbrake B/B, reverse clutch R/C, low and reverse brake LR/B, and overrunclutch OR/C are all held deactivated because they are activated on theline pressure coming out of these ports of the manual selector valve 36.This renders the power train shown in FIG. 2 in the neutral state wherethe power transmission is impossible.

D RANGE

When the driver places the manual selector valve 36 at the D range, theautomatic shift is effected in the following manner:

(First Speed Ratio)

With the manual selector valve 36 placed at the D range, the linepressure from the circuit 78 is supplied to the port 36 D as shown bythe TABLE 2. As a D range pressure, the line pressure from the port 36 Dis supplied via the circuit 106 to the port 38g of the first shift valve38, the port 40g of the second shift valve 40 and the forward clutchcontrol valve 46, and it also supplied via the circuit 112 to the port56c of the shuttle valve 56 and the port 58g of the overrun clutchcontrol valve 58.

When the vehicle is at a standstill with the manual selector valve 36placed at the D range, the computer causes the first shift solenoid 42and the second shift solenoid 44 to be turned ON, causing the firstshift valve 38 and the second shift valve 40 to assume the positions asillustrated by the right halves thereof, respectively, as viewed in FIG.1B. As a result, the high clutch H/C is allowed to communicate via thecircuit 118 and the port 40f with the drain port 40e, and thus it isdeactivated. The band brake B/B is deactivated because the second speedratio servo apply chamber 2S/R, the third speed ratio servo releasechamber 3S/R and the fourth speed ratio servo apply chamber 4S/Acommunicate with the same drain port 40e. The second speed ratio servoapply chamber 2S/A is allowed to communicate via the circuit 113, theport 38f, the port 38k, the circuit 118 and the port 40f with the drainport 40e. The third speed ratio servo release chamber 3S/R is allowed tocommunicate via the circuit 121, the circuit 118 and the port 40f withthe drain port 40e. The fourth speed ratio servo apply chamber 4S/A isallowed to communicate with the same drain port 40e in the mannerhereinafter described. As long as the engine output torque is higherthan a certain level, since it rises in proportion to the engine outputtorque, the D range pressure (i.e., line pressure) supplied to the port56c of the shuttle valve 56 keeps the spool 56b at the position asillustrated by the right half thereof as viewed in FIG. 1C where thepilot pressure within the circuit 79 is admitted via the circuit 134 tothe overrun clutch control valve 58, urging this valve 58 to theposition as illustrated by the right half thereof as viewed in FIG. 1C.The overrun clutch control valve 58 is kept at the position asillustrated by the right half thereof as viewed in FIG. 1C even when theengine output torque is not higher than the certain level and theshuttle valve 56 stays in the position as illustrated by the left halfthereof as viewed in FIG. 1C because unless there is no engine brakecommand later described, the third shift solenoid 60 is turned ON underthe control of the computer in order to cause the control pressuredirected to the overrun clutch control valve 58 via the circuits 109 and134 as high as the above mentioned pilot pressure. Under theseconditions, therefore, the fourth speed ratio servo apply chamber 4S/Ais allowed to communicate via the circuit 135, the ports 58d, 58h, thecircuit 116, the ports 38h, 38l, the circuit 118 and the port 40f withthe drain port 40e.

Besides, the reverse clutch R/C is drained via a circuit 128 at the port36 R, and thus it is deactivated. The low and reverse brake LR/B isdrained in the manner described hereinafter and thus deactivated. Thatis, the shuttle ball 127, which has its outlet port connected to thecircuit 130 communicating with the low and reverse brake LR/B, has oneof its two inlet ports connected to the circuit 128 that is drained asdescribed and the other inlet port connected to the circuit 126 that isin turn connected via the port 38d, 38j, the circuit 125, the ports 40d,40j, and the circuit 131 to the I range pressure reduction valve 54.Since it is not subject to pressure from the port 36 I of the manualselector valve 36 and thus left in the position as illustrated by theright half thereof as viewed in FIG. 1C, the low and reverse brake LR/Bis drained at the port 36 I via the circuit 132 and deactivated. Theoverrun clutch OR/C is allowed to communicate via the circuit 138, checkvalve 139 and the port 58f with the drain port 58e since the overrunclutch control valve 58 is held in the position as illustrated by theright half thereof as viewed in FIG. 1C for the previously mentionedreason, and thus it is deactivated.

As previously described, there exists no hydraulic pressure within thecircuit 113 that leads to the second speed ratio servo apply chamber2S/A so that the shuttle valve 32 stays in the position as illustratedby the lower half thereof as viewed in FIG. 1A because the chamber 32cis connected to the circuit 113 via the circuit 115. With the shuttlevalve 32 in the position as illustrated by the lower half thereof, thepilot pressure within the circuit 79 is supplied via the circuit 95 tothe chamber 30d, urging the lock-up control valve 30 to the position asillustrated by the right half thereof as viewed in FIG. 1A, renderingthe torque converter 3 operable in the converter state. In thisposition, the shuttle valve 32 allows the supply of the control pressurewithin the circuit 97 to the chamber 46a via the circuit 96. With theduty cycle of the solenoid 34 appropriately controlled, this controlpressure is adjusted such that the forward clutch control valve 46 iscontrolled in the following manner.

That is, 100% is set as the duty cycle of the solenoid 34 to cause thecontrol pressure supplied to the chamber 46a to be zero unless manualoperation for starting the vehicle is performed even under the D rangecondition. (The manual operation for starting the vehicle involvesdepression of an accelerator pedal with a foot brake released when thevehicle speed is zero.) This causes the forward clutch control valve 46to assume the position as illustrated by the right half thereof asviewed in FIG. 1A where the D range pressure supplied to the circuit 106is blocked and thus prevented from reaching the forward clutch F/C,leaving the forward clutch F/C deactivated. Under this condition, sincethe high clutch H/C, band brake B/B, reverse clutch R/C, low and reversebrake LR/B, and overrun clutch OR/C are deactivated, too, as previouslydescribed, the automatic transmission stays in neutral unless the manualoperation for starting the vehicle is performed even when the D range isselected in the automatic transmission. As a result, the occurrences ofcreep and select shock (i.e., a N-D select shock) are prevented.

Upon performing the manual operation for starting the vehicle, thecomputer starts decreasing the duty cycle of the solenoid 34 at agradual rate until it finally become zero. This causes the gradualincrease in the control pressure supplied to the chamber 46a untilfinally it increases to the same level as the pilot pressure. Thisgradual increase in the control pressure causes the spool 46b of theforward clutch control valve 46 to switch its position from the positionas illustrated by the right half thereof to the position as illustratedby the left half thereof at the corresponding gradual rate, causing agradual increase in hydraulic pressure supplied to the forward clutchF/C through the circuit 105 until the hydraulic pressure therein becomesas high as the D range pressure (i.e., line pressure) from the circuit106. Thus, the forward clutch F/C is gradually activated until theautomatic transmission establishes the first speed ratio in cooperationwith the activations of the forward one-way clutch FO/C and low one-wayclutch LO/C as shown in the Table 1, allowing the vehicle to move from astandstill. During this vehicle's starting operation, since the workinghydraulic pressure of the forward clutch F/C gradually rises, theactivation of the forward clutch F/C progresses at a predetermined speedin cooperation with the throttle effect due to the one-way orifice, thuspreventing start-up shock.

(Second Speed Ratio)

When the vehicle has reached the running state where the second speedratio is to be established as a result of an increase in the vehiclespeed, the computer switches the state of the first shift solenoid 42 tothe OFF state in accordance with the pattern shown in TABLE 3, causingthe spool 38b of the first shift valve 38 to switch to the position asillustrated by the left half thereof as viewed in FIG. 1B. As a result,the first shift valve 38 now allows the circuit 126 to communicate withthe drain port 38e, keeping on draining the circuit 126, and now allowsthe port 38h to communicate with the port 38i to keep on draining thecircuit 116 via the ports 38h, 38i, and the port 52c of the 4-2 sequencevalve 52 (This valve assumes the position as illustrated by the righthalf thereof as viewed in FIG. 1B under the condition where the thirdspeed ratio servo release chamber 3S/R is not supplied with hydraulicpressure.) at the drain port 52d. On the other hand, the first shiftvalve 38 allows the circuit 113 to communicate with the circuit 106,supplying the second speed ratio servo apply chamber 2S/A with the Drange pressure via the circuit 113, activating the band brake B/B. Withthe forward clutch F/C kept activated and the forward one-way clutchbeing activated, the activation of the band brake B/B causes theautomatic transmission to shift to the second speed ratio as readily beunderstood from TABLE 1.

During this upshift operation from the first to second speed ratio, thehydraulic fluid pressure supplied to the second speed ratio servo applychamber 2S/A is increased at a gradual rate owing to the fluid flowrestriction due to the one-way orifice 114 and gradual upward movement,as viewed in FIG. 1B, of the accumulator piston 64a in response to thepressure build-up in the accumulator chamber 64e, thus alleviatingshocks inherent with this shifting operation. Besides, the effect ofthis shift shock alleviation is secured because the backup pressurewithin the chamber 64d acting upon the accumulator piston 64a isproportional to the engine output torque.

Since, as readily understood from TABLE 1, the D range pressure issupplied to the second speed ratio servo apply chamber 2S/A not onlyupon selecting the second speed ratio, but also selecting the thirdspeed ratio and the fourth speed ratio, the shuttle valve 32 keeps onstaying in the position as illustrated by the upper half thereof asviewed in FIG. 1A because the D range pressure is supplied via thecircuit 115 to the chamber 32c. This allows the supply of the pilotpressure from the circuit 79 to the chamber 46a of the forward clutchcontrol valve 46, keeping the forward clutch control valve 46 in theposition as illustrated by the left half thereof as viewed in FIG. 1A.Since the forward clutch control valve 46 does not effect pressureregulation when it assumes this position, the forward clutch F/C is keptin the fully activated state upon selecting any one of the second speedto fourth speed ratios. On the other hand, the chamber 30d of thelock-up control valve 30 is supplied with the control pressure from thecircuit 97 so that the state of the torque converter 3 can be switchedamong the converter state, slip control state, and lock-up state inaccordance with a predetermined pattern matched to the operatingconditions by controlling the duty cycle of the duty solenoid 34 underthe control of the computer.

(Third Speed Ratio)

When, subsequently, the vehicle has attained the running state where thethird speed ratio is to be established, the computer switches the stateof the second shift solenoid 44 to the OFF state, too, in accordancewith the pattern shown in TABLE 3, causing the spool 40b of the secondshift valve 40 to switch to the position as illustrated by the left halfthereof as viewed in FIG. 1B. As a result, the D range pressure admittedto the port 40g is now supplied via the port 40f and circuit 118 to thehigh clutch H/C for activation thereof, In the process, the hydraulicfluid passes through the one-way orifice 120 unrestricted and thenthrough the one-way orifice 119 restricted. On the other hand, thishydraulic fluid pressure passes through the branch circuit 121 off thecircuit 118 and through the one-way orifice 122 unrestricted to thethird speed ratio servo release chamber 3S/R, deactivating the bandbrake B/B. The hydraulic fluid pressure supplied to the third speedratio servo release chamber 3S/R is applied to the chamber 52e of the4-2 sequence valve 52 via the circuit 124, urging the spool 52b forupward movement toward the position as illustrated by the left halfthereof. Even though this upward movement causes the port 52c tocommunicate with the port 52f after separating it from the drain port52d, since the second shift valve 40 connects this port 52c to the drainport 40i, the circuit 116 continues to be drained. Thus, deactivation ofthe band brake B/B with the high clutch H/C and the forward one-wayclutch FO/C held activated causes the automatic transmission to shift tothe third speed ratio.

During this upshift operation from the second speed ratio to the thirdspeed ratio, the hydraulic fluid pressure supplied to the high clutchH/C and the third speed ratio servo release chamber 3S/R graduallyincreases owing to the fluid flow restriction due to the one-way orifice122 and gradual upward movement, as viewed in FIG. 1B, of theaccumulator piston 66a against the line pressure within the chamber 66c,thus preventing the occurrence of shocks inherent with this shiftingoperation.

(Fourth Speed Ratio)

When, subsequently, the vehicle has reached the running state where thefourth speed ratio is to be established, the computer switches the stateof the first shift solenoid 42 to the ON state in accordance with thepattern shown in TABLE 3, causing the spool 38b of the first shift valve38 to switch to the position as illustrated by the position asillustrated by the right half thereof as viewed in FIG. 1B. This causesthe first shift valve 38 to switch the port connection such that eventhough it disconnects the circuit 113 leading to the second speed ratioservo apply chamber 2S/A from the D range pressure circuit 106, itallows the circuit 113 to communicate via the port 38k with the circuit118 in order to continue the supply of the D range pressure to thesecond speed ratio servo apply chamber 2S/A, whereas even though itdisconnects the circit 126 from the drain port 38e, it allows thecircuit 126 to communicate via the port 38j with the circuit 125 thatcommunicates with the drain port 40e in order to cause the circuit 126to continue to be drained. In this position, the first shift valve 38allows the circuit 116 to communicate with the circuit 118 via the ports38h and 38l, supplying the D range pressure via the circuit 118, circuit116, ports 58h and 58d, circuit 135 and one-way orifice 136 to thefourth speed ratio servo apply chamber 4S/A, thus switching the state ofthe forward clutch F/C to the activated state. Thus, activating the bandbrake B/B with the forward clutch F/C and high clutch H/C held activatedcauses the automatic transmission to shift to the fourth speed ratio.

During this upshift operation from the third to fourth speed ratio, thefourth speed ratio select hydraulic fluid pressure (i.e., the highestspeed ratio select pressure) supplied to the fourth speed ratio servoapply chamber 4S/A increases at a gradual rate owing to the fluid flowrestriction due to the one-way orifice 136 and gradual upward movement,as viewed in FIG. 1C, of the accumulator piston 68a in response to thepressure build-up in the accumulator chamber 68c, thus alleviatingshocks inherent with this shifting operation. Besides, the effect ofthis shift shock alleviation is secured because the backup pressurewithin the chamber 68c acting upon the accumulator piston 68a isproportional to the engine output torque.

Besides, this pressure (i.e., the fourth speed ratio select pressure)supplied to the fourth speed ratio servo apply chamber 4S/A passes tothe chamber 66d of the accumulator 66. This results in switching thecapacity of the accumulator 66 to a value meeting the demand for theupshift from the second speed ratio to the fourth speed ratio, thisvalue being different from the capacity required for the upshift fromthe second speed ratio to the third speed ratio. As a result, shiftshock alleviation takes place effectively even during the upshift fromthe second speed ratio to the fourth speed ratio jumping the third speedratio.

(4-3 Downshift)

When, during running with the fourth speed ratio, the vehicle hasreached the running state where the third speed ratio is to beestablished, the computer switches the state of the first shift solenoid42 to the OFF state in accordance with the pattern shown in TABLE 3,causing the first shift valve 38 to switch to the position asillustrated by the left half thereof as viewed in FIG. 3B to have itassume the same position as it assumes upon selecting the third speedratio. As a result, the hydraulic pressure supplied to the fourth speedratio servo apply chamber 4S/R is depressurized quickly because thehydraulic fluid passes through the one-way orifice 136 unrestricted tobe discharged at the drain port 40i, causing the downshift to the thirdspeed raio to take place.

(4-2 Downshift)

When, during running with the fourth speed ratio, the vehicle hasreached the running state where the second speed ratio is to beestablished, the computer switches the state of the first shift solenoid42 to the OFF state, thereby to switch the first shift valve 38 to theposition as illustrated by the left half thereof as viewed in FIG. 1B,and it switches the state of the second shift solenoid 44 to the ONstate, thereby to switch the second shift valve 40 to the position asillustrated by the right half thereof as viewed in FIG. 1B. Even thoughthis movement of the first shift valve 38 switches the connection of thecircuit 113 leading to the second speed ratio servo apply chamber 2S/Ato the circuit 106 from the circuit 118, the first shift valve 38continues to supply hydraulic pressure to the second speed ratio servoapply chamber 2S/A. The switching of the second shift valve 40 resultsin disconnecting the circuit 118 from the D range pressure circuit 106and connecting it to the drain port 40e. As a result, the activatingpressure supplied to the high clutch H/C is eliminated by dischargingthe hydraulic fluid via the circuit 118 at the drain port 40e afterhaving passed through the one-way orifice 119 unrestricted and beingsubject to flow restriction by the one-way rifice 120, and the pressurewithin the third speed ratio servo release chamber 3S/R is eliminated bydischarging the hydraulic fluid via the circuit 121 and then through thesame path at the drain port 40e after having being subject to flowrestriction by the one-way orifice 122. The pressure within the thirdspeed ratio servo release chamber 3S/R is delivered via the circuit 124to the 4-2 sequence valve 52. This 4-2 sequence valve 52 is responsiveto this pressure and assumes the position as illustrated by the lefthalf thereof as viewed in FIG. 1B where the port 52c that is connectedto the circuit 116 via the ports 38i and 38h is disconnected from thedrain port 52d and allowed to communicate with the port 52f until thispressure is substantially eliminated. This prevents the pressure withinthe fourth speed ratio servo apply chamber 4S/A connected to the circuit116 from being drained and thus it is maintained until the third speedratio servo release chamber 3S/R is drained. During this transition, thepressure within the fourth speed ratio servo apply chamber 4S/R issupplied via the circuit 116 to the 4-2 relay valve 50, holding thisvalve to the position as illustrated by the right half thereof as viewedin FIG. 1B. This allows the circuit 113 leading to the second speedratio servo apply chamber 2S/A to communicate with the fourth speedratio servo apply chamber 4S/A via the ports 50f, 50c, circuit 117,ports 40k, 40h, 52f, 52c, 38i, 38h, circuit 116, ports 58h, 58d, andcircuit 135, thus keeping the pressure within the fourth speed ratioservo apply chamber 4S/R.

When the pressure within the third speed ratio servo release chamber3S/R is drained, the 4-2 sequence valve 52 shifts to the position asillustrated by the right half thereof as viewed in FIG. 1B where thepressure within the fourth speed ratio servo apply chamber 4S/Aconnected to the circuit 116 is drained at the drain port 52d. Thedraining of the pressure within the circuit 116 causes the 4-2 relayvalve 50 to shift to the position as illustrated by the left halfthereof as viewed in FIG. 1B, allowing the pressure within the circuit117 to be drained at the drain port 50d. Thus, during this 4-2 downshiftoperation, the pressure within the fourth speed ratio servo applychamber 4S/A is drained after the pressure within the high clutch H/Chas been drained so that the 4-2 downshift skipping the third speedratio is assured by preventing the former pressure from being drainedprior to the drainage of the latter pressure which would cause the 4-3-2downshift.

(3-2 Downshift)

When, during running with the third speed ratio, the vehicle has reachedthe running state where the second speed ratio is to be established, thecomputer switches the state of the second solenoid 44 to the ON state tocause the second shift valve 40 to shift to the position as illustratedby the right half thereof as viewed in FIG. 1B in accordance with thepattern shown in TABLE 3. Even though this switches the connection ofthe port 40h from the drain port 40i to 40k, the fourth speed ratioservo apply chamber 4S/A is kept in the depressurized state regardlessof the position of the 4-2 sequence valve 52 because, during runningwith the third speed ratio, the circuit 116 connected to the fourthspeed ratio servo apply chamber 4S/A is depressurized to cause the 4-2relay valve 50 to assume the position as illustrated by the left halfthereof where the circuit 117 is connected to the drain port 50d andthus the port 52f connected to the circuit 117 is caused to serve as adrain port.

The above mentioned shift of the second shift valve 40 causes thecircuit 118 to communicate with the drain port 40e, draining thepressure within the high clutch H/C and the pressure within the threespeed ratio servo release chamber 3S/R in the same manner as describedin connection with the 4-2 downshift. This results in a downshift fromthe third speed ratio to the second speed ratio. As will be describedhereinafter, the timing at which the pressure within the third speedratio servo release chamber 3S/R varies depending on the operatingconditions of the engine such that smooth 3-2 downshift takes place.

That is, when the engine output torque is below a predetermined value,the D range pressure (i.e., line pressure) applied to the port 56c ofthe shuttle valve 56 is low corresponding in magnitude to the engineoutput torque, allowing the shuttle valve 56 to assume the position asillustrated by the left half thereof as viewed in FIG. 1C, allowing thechamber 48e of the 3-2 timing valve 48 to communicate via the circuit133 and port 56e with the drain port 56f, thus allowing the 3-2 timingvalve 48 to assume the position as illustrated by the left half thereofas viewed in FIG. 1C. Thus, when the engine output torque is low, thethird speed ratio servo release chamber 3S/R is depressurized quicklybecause the hydraulic fluid is discharged not only through the one-wayorifice 122, but also through the orifice 48f, When the engine outputtorque is above the predetermined level, the D range pressure (i.e.,line pressure) applied to the port 56c of the shuttle valve 56 is highcorresponding to the increased engine output torque, urging the shuttlevalve 56 to the position as illustrated by the right half thereof asviewed in FIG. 1C, connected the circuit 133 to the circuit 109, thusrendering the 3-2 timing valve 48 shiftable in accordance with thecontrol pressure produced within the circuit 109. The computer switchesthe state of the third shift solenoid 60 to the ON state when the engineoutput torque is above the predetermined level and the vehicle speed ishigher than a predetermined speed, causing the control pressure toincrease to a value as high as the pilot pressure. This causes the 3-2timing valve 48 to shift to the position as illustrated by the righthalf thereof as viewed in FIG. 1C, depressurizing the third speed servorelease chamber 3S/R slowly by discharging the hydraulic fluid throughthe one-way orifice 122 only.

(2-1 Downshift)

When, during running with the second speed ratio, the vehicle reachesthe running state where the first speed ratio is to be established, thecomputer switches the state of the shift solenoid 42 to the ON state,thus switching the first shift valve 38 to the position as illustratedby the right half thereof as viewed in FIG. 1B in accordance with thepattern shown in TABLE 3. This causes the circuit 113 leading to thesecond speed ratio servo apply chamber 2S/A to disconnected from the Drange pressure circuit 106, but to be connected to the circuit 118 viathe ports 38f and 38k. Since the circuit 118 is connected to the drainport 40e by the second shift valve 40, the pressure within the secondspeed ratio servo apply chamber 2S/A is depressurized quickly bydischarging the hydraulic fluid passing through the one-way orifice 114,thus effecting a quick downshift from the second speed ratio to thefirst speed ratio.

(Overdrive Inhibition)

When the driver turns ON an OD inhibitor switch, not illustrated,disposed within the reach of the driver, wishing the engine brake to beeffected with the third speed ratio and wishing no upshift to the fourthspeed ratio (overdrive), the output of the OD switch causes the computerto set the ON/OFF state of the first and second shift solenoids 42 and44 in accordance with the pattern shown in TABLE 3 so as to provide aspeed ratio suitable for the driving state such that the fourth speedratio will not be established. In this case, the automatic transmissionis allowed to perform automatic shift between the first, second andthird speed ratios in the similar manner as in the D range.

With the third speed ratio to be selected, the computer switches thestate of the third shift solenoid 60 to the OFF state, bringing down thecontrol pressure within the circuit to zero. When, under this condition,the engine torque is low (this is the case where the engine brake isneeded) and accordingly the D range pressure (i.e., line pressure)applied to the port 56c of the shuttle valve 56 is low, thus failing tourge the shuttle valve 56 to the position as illustrated by the righthalf thereof as viewed in FIG. 1C, leaving it in the position asillustrated by the left half thereof as viewed in FIG. 1C, the controlpressure within the circit 109 brought down to zero is admitted to theoverrun clutch control valve 58 via the circuit 134. However, thisadmission of the control pressure to the chamber 58v does not cause theoverrun clutch control valve 58 to shift from the position asillustrated by the left half thereof as viewed in FIG. 1C. Since theoverrun clutch control valve 58 remains in the position as illustratedby the left half thereof, the D range pressure within the circuit 112 isnow supplied via the circit 137 and the overrun clutch pressurereduction valve 62 to the overrun clutch OR/C, thus activating the same.This activation of the overrun clutch OR/C causes the transmission toeffect the engine brake running with the third speed ratio as will beunderstood from the TABLE 1. During this engine brake running, sincethere is no hydraulic pressure at the port 36 II of the manual selectorvalve 36, the overrun clutch pressure reduction valve 62 effectspressure regulation to reduce the activating pressure supplied to theoverrun clutch OR/C, matching the capacity thereof to a value required,thus causing substantial reduction in shocks upon shifting to enginebrake operation. When the engine output torque is high or large (this isthe case where engine brake is not required) and thus the D rangepressure applied to the port 56c of the shuttle valve 56 is high enoughto urge the shuttle valve 56 to assume the position as illustrated bythe right half thereof, allowing the pilot pressure within the circuit79 to be admitted via the circuit 134 to the overrun clutch controlvalve 58 to urge the same to the position as illustrated by the righthalf thereof as viewed in FIG. 1C. This position of the overrun clutchcontrol valve 58 causes the circuit 138 to be drained always, thusrendering the overrun clutch OR/C deactivated, causing no engine braketo be effected. The hydraulic fluid to be discharged from the overrunclutch OR/C in this case is quickly discharged at the drain port 58e viathe check valve 139.

II Range

When the driver places the spool 36a of the manual selector valve 36 atthe II range, wishing the engine brake running with the second speedratio, the manual valve allows the line pressure within the circuit 78to output from the port 36 II, too, in accordance with the pattern shownin TABLE 2. Under this condition, the pattern of distribution of thepressure from the port 36 D is quite the same as it is upon selectingthe D range. The computer switches the states of the first and secondshift solenoids 42 and 44 in accordance with the pattern shown in TABLE3 for selecting the first speed ratio or the second speed ratio, thuscausing the automatic transmission to shift between the first speedratio and the second speed ratio.

The pressure (II range pressure) from the port 36 II of the manualselector valve 36 reaches the port 62c of the overrun clutch pressurereduction valve 62 via the circuit 140. The II range pressure from thecircuit 140 reaches also to the chamber 56g of the shuttle valve 56,locking this valve to the position as illustrated by the left halfthereof as viewed in FIG. 1C. With the shuttle valve 56 locked to thisposition, the control pressure within the circuit 109 is supplied viathe circuit 134 to the chamber 58c of the overrun clutch control valve58. In this state, the computer brings down the control pressure to zeroduring running with the second speed ratio by switching the state of thethird shift solenoid 60 to the OFF state, causing the overrun clutchcontrol valve 58 to assume the position as illustrated by the left halfthereof as viewed in FIG. 1C. This causes the D range pressure withinthe circuit 112 to be supplied to the overrun clutch OR/C via theoverrun clutch pressure reduction valve 62 and the circuit 137, thuscausing the automatic transmission to effect engine brake with thesecond speed ratio.

Under this condition, the overrun clutch pressure reduction valve 62does not effect pressure reduction since it is locked as mentionedabove, allowing the capacity of the overrun clutch OR/C to increase to avalue matched to the requirement, thus preventing engine brake frombecoming ineffective owing to the shortage in capacity.

During running with the first speed ratio selected, the computer turnsON the third shift solenoid 60, allowing an increase in the controlpressure to the level as high as the pilot pressure, causing the overrunclutch control valve 58 to stay in the position as illustrated by theright half thereof as viewed in FIG. 1C. This results in discharge ofthe hydraulic fluid from the overrun clutch OR/C via the check valve 139and port 58f at the drain port 58e, deactivating the overrun clutchOR/C, thus establishing the same state as is the case for running withthe first speed ratio during the D range.

I Range

When the driver places the spool 36a of the manual selector valve 36wishing the engine brake running with the first speed ratio, the linepressure within the circuit 78 is allowed to output from the ports 36 D,36 II and 36 I in accordance with the pattern shown in TABLE 2. Thepattern of distribution of the pressure from the port 36 D is quite thesame as it is upon selecting the D range. The computer switches thestates of the first and second shift solenoids 42 and 44 in accordancewith the pattern shown in TABLE 3 for selecting the first speed ratio orthe second speed ratio, thus causing the automatic transmission to shiftbetween the first speed ratio and the second speed ratio. There arecases where the second speed ratio is established despite the fact the Irange is selected because it is necessary to prevent the occurrence ofthe overrun of the engine which would be caused if the first speed ratiowere established immediately after selecting the I range. In thissituation, the second speed ratio is established and maintained untilthe vehicle speed drops sufficiently so that the downshift to the firstspeed ratio does not cause any excessive rotation of the engine.

The pressure from the port 36 II of the manual selector valve 36 issupplied to the shuttle valve 56 and the overrun clutch pressurereduction valve 62 in the same manner as the case upon selecting the IIrange, thereby to hold these valves to the position as illustrated bythe left halves thereof, respectively. This allows the control pressurewithin the circuit 109 to switch the state of the overrun clutch controlvalve 58. With the I range selected, the computer turns OFF the thirdshift solenoid 60 to bring down the control pressure to zero, allowingthe overrun clutch control valve 58 to assume the position asillustrated by the left half thereof as viewed in FIG. 1C, thus allowingthe D range pressure within the circuit 112 to keep on activating theoverrun clutch OR/C.

The pressure from the port 36 I of the manual selector valve 36 arrivesvia the circuit 132 at the I range pressure reduction valve 54 where itis reduced to the certain value in the manner described before andoutput to the circuit 131. The pressure within the circuit 131 isallowed to appear in the circuit 125 because the second shift valve 40is held in the position as illustrated by the right half thereofregardless of which the first speed ratio or the second speed ratio isestablished. The first shift valve 38, on the other hand, stays in theposition as illustrated by the left half thereof as viewed in FIG. 1Bduring running with the second speed ratio in accordance with thepattern shown in TABLE 3 where it cuts off the pressure within thecircuit 125 and drains the pressure within the circuit 126 at the drainport 38e. As a result, the circuit 130 leading to the low and reversebrake LR/B is allowed to communicate via the shuttle ball 125 and thecircuit 126 with the drain port 38e, causing deactivation of the low andreverse brake LR/B. This allows the engine brake running with the secondspeed ratio owing to the activation of the overrun clutch OR/C.

The first speed ratio is established after selecting the I range uponthe vehicle speed dropping low enough not to cause excessive rotation ofthe engine. With this first speed ratio, the first shift valve 38 is inthe position as illustrated by the right half thereof as viewed in FIG.1B where the circuit 125 is allowed to communicate with the circuit 126,thus supplying the pressure within the circuit 125 via the circuit 126,shuttle ball 127 and circuit 130 to the low and reverse brake LR/B toactivate the same. This results in establishment of engine brake runningwith the first speed ratio because the overrun clutch OR/C is activated.

During engine brake running with the first or second speed ratio, theoverrun clutch pressure reduction valve 62 is locked to the position asillustrated by the left half thereof as viewed in FIG. 1C, causing nopressure reduction to take place. This causes the capacity of theoverrun clutch OR/C to be maintained sufficiently high enough to thevalue demanded, thus preventing the engine brake from becomingineffective due to the slip therein. Since during engine brake runningwith the first speed ratio the pressure admitted to the low and reversebrake LR/B is reduced to the certain value by means of the I rangepressure reduction valve 54, the capacity of the low and reverse brakeis adjusted to the value demanded, thus allowing the downshift to thefirst speed ratio to take place without substantial shocks.

R Range

When the driver places the spool 36a of the manual selector valve 36 atthe R range wishing the reverse travel, the line pressure within thecircuit 78 is allowed to output from the port 36 D only in accordancewith the pattern shown in TABLE 2. The pressure from the port 36 R(reverse select pressure) is supplied via the circuit 128 and theone-way orifice 129 where it is subject to fluid flow restriction to thereverse clutch R/C to activate the same and at the same time it issupplied to the chamber 68d of the accumulator 68. The pressure withinthe circuit 128 is supplied via the circuit 130 to the low and reversebrake LR/B to activate the same pushing through the shuttle ball 127. Asa result, the automatic transmission selects the reverse drive as shownin TABLE 1.

During this transition, the pressure supplied to the reverse clutch R/Cincreases at a gradual rate to cause the engagement of the reverseclutch R/C to progress at a predetermined gradual rate because thehydraulic fluid to be supplied to the reverse clutch R/C is restrictedby the one-way orifice 129 and then pushes down the stepped piston 68aof the accumulator upon selecting the R range (this stepped piston beingshifted to the position as illustrated by the left half thereof asviewed in FIG. 1C upon placing the spool 36a of the manual selectorvalve 36 at the R range), thus alleviating shocks (a N-R select shock)taking place upon selecting the R range from the N range.

Referring to FIG. 1C, the accumulator 68 is designed to alleviate selectshock occurring when the R range is selected in the transmission. Thus,the setting of the spring 68b is such that the downward movement of thepiston 68a against the spring 68b begins immediately after the reversedrive select pressure has been applied to the chamber 68d with the othertwo chambers 68c and 68e depressurized. In other words, the spring forceof the spring 68b is low enough to allow such downward movement of thepiston.

Let us now consider why the shift shock is effectively alleviated whenthe transmission shifts from the third speed ratio to the fourth speedratio despite the fact that the piston 68a of the accumulator will notstart moving upwards from the position as illustrated by the right halfthereof as viewed in FIG. 1C until the pressure within the chamber 68ebuilds up and becomes high enough to cause the piston 68a to expand thechamber 68e. As will be understood from the engine specific torqueversus throttle opening degree characteristic curve (beta) shown in FIG.6, the engine specific torque starts rising after the throttle openingdegree has become greater than a sufficiently great value. Thus, even ifthe piston 68a of the accumulator 68 will not start moving upwards fromthe position as illustrated by the right half thereof as viewed in FIG.1C during the pressure build-up within the chamber 68e of theaccumulator 68 with low throttle opening degrees, the shift shock whichwill occur when the engine specific torque rises during operatingcondition with relartively great throttle opening degrees can beeffectively alleviated. This explains why the shift shock when thetransmission shifts from the third speed ratio to the fourth speed ratiocan be effectively alleviated despite the fact the spring force of thespring 68b is set as equal to a value sufficiently small enough toeffectively alleviate the N-D select shock.

The second embodiment is described hereinafter along with FIG. 4.

In the first embodiment, the line pressure is supplied as a back-uppressure to the chamber 68c from the line pressure circuit 78 throughthe circuit 111 via the accumulator control valve 70 and this back-uppressure is not used to activate the forward clutch F/C that is to bekept engaged for forward drive with every speed ratio (see TABLE 1). Theforward clutch control valve 46 as shown in FIG. 1A is provided foralleviating the N-D select shock. In the second embodiment shown in FIG.4, a back-up pressure chamber 68c of an accumulator 68 is connected to acircuit 105 at a portion between the one-way orifice 107 and the forwardclutch F/C, see FIG. 1C, through a circuit 141 so as to permit theforward drive select pressure to be supplied to the back-up pressurechamber 68c. Since the spring force of a spring 68b is set similarly tothe first embodiment, a piston 68a of the accumulator 68 moves downwardsfrom the position as illustrated by the left half thereof immediatelyafter the pressure build-up in the back-up pressure chamber 68c. Thisassures effective alleviation of the N-D select shock.

What is claimed is:
 1. An apparatus comprising:an automatic transmissionwhich is shiftable to a plurality of speed ratios including apredetermined speed ratio other than a first speed ratio and a secondspeed ratio, the predetermined speed ratio haing a reduction ratiosmaller than a reduction ratio of the first or second speed ratio, theautomatic transmission being shiftable from neutral to a reverse driveratio, and a control system comprising: an accumulator including apiston defining three chambers which are selectively expandable involume in response to movement of said piston, and resilient meansacting on said piston in assisting movement of said piston in one of twodirections and yieldably resisting movement of said piston in the otherdirection which is opposite to said one direction, said three chambersbeing comprised of a first chamber, a second chamber and a thirdchamber; means for supplying a forward drive select hydraulic fluidpressure to said first chamber to act on said piston to urge said pistonagainst said resilient means in such a manner as to decrease the volumeof said second chamber when a forward drive range is selected in theautomatic transmission and discharging hydraulic fluid from said firstchamber when a reverse drive range is selected; a first friction elementadapted to be engaged when the transmission upshifts to thepredetermined speed ratio when said forward drive range is selected;means for initiating an upshift to the predetermined speed ratio bysupplying a predetermined speed select hydraulic fluid pressure to saidfirst friction element; said second chamber of said accumulatorcommunicating with said first friction element to allow saidpredetermined speed select hydraulic fluid pressure to act on saidpiston in such a manner as to assist the action of said resilient meansto cause said piston to move in said one direction so as to expand thevolume of said second chamber, allowing gradual build-up of saidpredetermined speed select hydraulic fluid pressure supplied to saidfirst friction element; a second friction element adapted to be engagedwhen said reverse drive range is selected; and means for initiating ashift from the neutral to the reverse drive ratio by supplying a reversedrive range select hydraulic fluid pressure to said second frictionelement when said reverse drive range is selected in the automatictransmission; said third chamber of said accumulator communicating withsaid second friction element to allow said reverse drive range selecthydraulic fluid pressure to act on said piston against the action ofsaid resilient means in said second direction so as to expand the volumeof said third chamber, allowing gradual build-up of said reverse driverange select hydraulic fluid pressure supplied to said second frictionelement.
 2. A control system as claimed in claim 1, wherein said firstchamber is a back-up pressure chamber.
 3. An apparatus as claimed inclaim 2, wherein said predetermined speed ratio is a fourth speed ratioand said first friction element is a band brake that is to be applied ineffecting an upshift to the second speed ratio and said fourth speedratio.
 4. An apparatus as claimed in claim 3, wherein said secondfriction element is a rear clutch to be engaged upon selecting saidreverse drive range.
 5. An apparatus as claimed in claim 4, including athird friction element which is to be engaged upon selecting saidforward drive range, and said third friction element communicates withsaid first chamber.
 6. An apparatus as claimed in claim 5, wherein saidthird friction element is a forward clutch.
 7. An apparatuscomprising:an automatic transmission for an automotive vehicle having anengine with a throttle which opens in degrees, the automatictransmission being shiftable between a plurality of forward speed ratiosincluding a predetermined speed ratio other than a first speed ratio anda second speed ratio, the automatic transmission being shiftable fromneutral to a reverse drive ratio, the predetermined speed ratio having areduction ratio smaller than a reduction ratio of the first or secondspeed ratio, and a control system comprising: a pressure regulator valvemeans for generating a servo actuating hydraulic fluid pressure which isvariable with the degree of throttle opening; a first friction elementadapted to be engaged when the transmission shifts to the second speedratio and when the transmission shifts to the predetermined speed ratiowhen a forward drive range is selected; shift means for initiating anupshift to the second speed ratio at a shift point where the degree ofthrottle opening is relatively small by supplying said servo actuatinghydraulic fluid pressure to said first friction element as a secondspeed select pressure and for initiating an upshift to the predeterminedspeed ratio at another shift point where the degree of throttle openingis relatively large by supplying said servo actuating hydraulic fluidpressure to said first friction element as a predetermined speed selectpressure; an accumulator incuding a piston defining three chambers whichare selectively expandable in volume in response to movement of saidpiston, and resilient means acting on said piston in assisting movementof said piston in one of two directions and yieldably resisting movementof said piston in the other direction which is opposite to said onedirection, said three chambers being comprised of a first chamber, asecond chamber and a third chamber; means for supplying said servoactuating hydraulic fluid pressure to said first chamber of saidaccumulator to act on said piston to urge said piston against saidresilient means in such a manner as to decrease the volume of saidsecond chamber when said forward drive range is selected and fordischarging hydraulic fluid from said first chamber when a reverse driverange is selected; said second chamber of said accumulator communicatingwith said first friction element to allow said predetermined speedselect pressure to act on said piston to assist the action of saidresilient means to cause said piston to move in said one direction so asto expand the volume of said second chamber, allowing gradual build-upof said predetermined speed select pressure supplied to said firstfriction element; a second friction element adapted to be engaged whenthe transmission shifts from the neutral to the reverse drive ratio uponselecting a reverse drive range; means for initiating a shift from theneutral to the reverse drive ratio by supplying said servo actuatinghydraulic fluid pressure to said second friction element upon selectingthe reverse drive range as a reverse drive select pressure; said thirdchamber of said accumulator communicating with said second frictionelement to allow said reverse drive select pressure to act on saidpiston to move said piston against the action of said resilient means insaid second direction so as to expand the volume of said third chamber,allowing gradual build-up of said reverse drive select pressure suppliedto said second friction element.
 8. An apparatus comprising:an automatictransmission for an automotive vehicle having an engine with a throttlewhich opens in degrees, the automatic transmission being shiftablebetween a plurality of forward speed ratios including a predeterminedspeed ratio other than a first speed ratio and a second speed ratio, theautomatic transmission being shiftable from neutral to a reverse driveratio, the predetermined speed ratio having a reduction ratio smallerthan a reduction ratio of the first or second speed ratio, and a controlsystem comprising: a pressure regulator valve means for generating aservo actuating hydraulic fluid pressure which is variable with thedegree of throttle opening; a first friction element adapted to beengaged when the transmission shifts to the second speed ratio and whenthe transmission shifts to the predetermined speed ratio when a forwarddrive range is selected, said first friction element including a servomotor means having a servo release chamber, a first servo apply chamberand a second servo apply chamber, said servo motor means being operativeto engage said first friction element when said first servo applychamber is pressurized with said servo release chamber depressurized andsaid second servo apply chamber depressurized, said servo motor meansbeing operative to release the engagement of said first friction elementwhen said servo release chamber is pressurized with said second servoapply chamber depressurized, said servo motor means being operative toengage said first friction element when said second servo apply chamberis pressurized; shift means for initiating an upshift to the secondspeed ratio at a shift point where the degree of throttle opening isrelatively small by supplying said servo actuating hydraulic fluidpressure to said first servo apply chamber of said servo motor means ofsaid first friction element as a second speed select pressure and forinitiating an upshift to the predetermined speed ratio at another shiftpoint where the degree of throttle opening is relatively large bysupplying said servo actuating hydraulic fluid pressure to said secondservo apply chamber of said servo motor means of said first frictionelement as a predetermined speed select pressure; an accumulatorincluding a piston defining three chambers which are selectivelyexpandable in volume in response to movement of said piston, andresilient means acting on said piston in assisting movement of saidpiston in one of two directions and yieldably resisting movement of saidpiston in the other direction which is opposite to said one direction,said three chambers being comprised of a first chamber, a second chamberand a third chamber; means for supplying said servo actuating hydraulicfluid pressure to said first chamber of said accumulator to act on saidpiston to urge said piston against said resilient means in such a manneras to decrease the volume of said second chamber when said forward driverange is selected and for discharging hydraulic fluid from said firstchamber when a reverse drive range is selected; said second chamber ofsaid accumulator communicating with said second servo apply chamber ofsaid servo motor means of said first friction element to allow saidpredetermined speed select pressure to act on said piston to assist theaction of said resilient means to cause said piston to move in said onedirection so as to expand the volume of said second chamber, allowinggradual build-up of said predetermined speed select pressure supplied tosaid first friction element; a second friction element adapted to beengaged when the transmission shifts from the neutral to the reversedrive ratio upon selecting a reverse drive range; and; means forinitiating a shift from the neutral to the reverse drive ratio bysupplying said servo actuating hydraulic fluid pressure to said secondfriction element upon selecting the reverse drive range as a reversedrive select pressure; said third chamber of said accumulatorcommunicating with said second friction element to allow said reversedrive select pressure to act on said piston to move said piston againstthe action of said resilient means in said second direction so as toexpand the volume of said third chamber, allowing gradual build-up ofsaid reverse drive select pressure supplied to said second frictionelement.